The present invention relates to magnetic resonance imaging and, in particular, to a dome-like radio frequency coil.
Radio frequency (RF) coils are used to excite and/or sense magnetic resonance (MR) signals for magnetic resonance imaging (MRI).
Birdcage resonators are a commonly used form of volume RF coil. A birdcage coil is typically cylindrically shaped with a ring element at each end connected with multiple axial elements. Ellipse-like and other shapes have also been used for the rings. Good radial RF field uniformity is achieved, but towards the ends of the cylinder, the RF field in the axial direction drops off substantially (e.g., 50 percent).
Improvements in uniformity of the RF field in the axial direction can be achieved by lengthening the coil. Unfortunately, this may be impractical for various reasons, including: the necessary space may not be available, particularly in the case of a head coil where the patient""s shoulders are in the way; or a larger coil may require too much power to be coupled into the patient.
Improved axial uniformity has also been produced by symmetrically locating a second ring near to each of the end rings. This structure does not however take advantage of the signal to noise ratio (SNR) improvements that may be achieved by having the coil conform better to the patient, for example, by getting close to the top of a patient""s head in a head coil.
Improved axial uniformity has also been achieved at one end of the coil by creating a dome-like structure by joining the axial elements together at that end instead of terminating them on an end ring. The dome may, for example, follow the general shape of a patient""s head and increase the RF field as the dome narrows, helping to compensate for the axial drop off in field that would occur otherwise. Unfortunately this can only improve the performance of one end of the coil. In addition, this convergence to a single point generates a very intense RF xe2x80x9chot spotxe2x80x9d. This hot spot can over rotate the local nuclei spins, resulting in the presence of xe2x80x9cblackxe2x80x9d spots in the resulting image.
A MRI coil having an axis and a first end and an opposite second end with respect to said axis includes a first ring element at the first end, a second ring element, a third ring element, a fourth ring element at the second end where the first ring element encompasses a smaller area than each of the second, third, and fourth ring elements. The coil also includes a plurality of axial elements connected between the first, second, third and fourth ring elements. The third and fourth ring elements are axially closer than the first and second ring elements.